Albion star Andrew Crofts has been told his season is over.
An MRI scan revealed he ruptured his cruciate ligament in yesterday's 1-0 win over Birmingham City.
He will undergo surgery this week and is expected to be sidelined for six months.
Crofts went down in agony during the second half, having earlier played a part in the goal by David Lopez which propelled Albion into the play-off places for the first time this season.
The Welsh international had been producing arguably his best ever run of form for the club and his absence is potentially a massive blow.
Head coach Oscar Garcia said: "I am very sorry for Crofty.
“He has been excellent this season and a key player. He wanted to play a big part in our season after missing so much of the last campaign through injury.
"Of course, he has already done that, with his performances and the key goals he has scored, and he will be a big loss - but we have a good squad with strength in depth to deal with this type of situation.”
Crofts will now target a return to full fitness in time for pre-season in July.